Exhaust emissions from internal combustion engines are a significant contributor to pollution in the environment. In particular, nitric oxide (NOx) emissions contribute to smog and acid rain. NOx, which includes both nitrogen oxide and nitrogen dioxide, is a byproduct of the combustion of fossil fuels, and diesel engines are regarded as a major generator of NOx.
To reduce the levels of NOx emitted into the atmosphere, the exhaust systems of internal combustion engines include catalytic converters that convert NOx into water and nitrogen. Catalytic converters in diesel engines do not work as well in reducing NOx in comparison to standard engines. This is due in part because diesel engines run cooler than standard engines, and the converters work better at higher temperatures. To address this shortcoming, the exhaust systems of diesel engines include a system for the Selective Catalytic Reduction (SCR) of NOx in which a urea solution is injected in the exhaust stream upstream of the catalytic converter. After injection into the exhaust stream, the urea solution evaporates and mixes with the exhaust stream. The urea decomposes and hydrolyzes into ammonia. NOx reacts with the thus generated ammonia in the presence of the catalyst and is catalytically reduced.
Many typical SCR systems utilize an injection nozzle that is solenoid or spring activated. Such injection nozzles typically only move between a single open and closed position and do not allow for precise control over the amount of urea that is injected into the exhaust stream. As a result, an insufficient amount of urea solution to neutralize the NOx in the exhaust stream may be introduced into the exhaust stream. To ensure that the NOx is fully neutralized, some systems inject a surplus of the urea solution into the exhaust stream, which may also be undesirable. Additionally, many urea injection systems use compressed air to assist in the injection and atomization process. The air acts as the delivery system and at the same time cleans out the delivery lines from unwanted urea thus mitigating any crystallization and clogging issues.
Accordingly, there still exists a need for a fluid injector having improved control over the amount of urea solution discharged from the injector, and the ability to shut off the flow and prevent clogging of the injector due to crystallization.